1. Field of the Invention
This invention relates to the field of fluid flow measurement and, more particularly, to miniaturized flow meters capable of measuring flow rates of tens to hundreds of microliters/minute (".mu.l/min").
2. Description of the Prior Art
In many fields, the measurement of small fluid flows is extremely important. Particularly in the fields of biological, medical sciences, analytical instrumentation, and drug delivery, the measurement of extremely small quantifies of fluids for dosimetry or titration, either of liquids or for gases, is a common problem. Frequently the quantity of fluid to be measured is almost in the nanoliter range. Often it is desirable, especially to avoid contamination in medical tests, that the flow meter be disposable and hence of low cost. Absolute accurate measurements are not always necessary, but precise and repeatable measurements are. For many of these applications, traditional flow meters, scaled down to smaller geometries, are infeasible. For example, flow meters that employ a pressure measurement principle become very difficult to implement.
An article by Gass, V., van der Schoot, B. H., and de Rooij, N. F. entitled "Nanofluid Handling by Micro-Flow-Sensor Based on Drag Force Measurements," published in the Proceedings of the IEEE Micro-Electronics Mechanical Systems conference held in Fort Lauderdale, Fla. during 1993, IEEE Catalog no. 93CH3265-6 ("the Gass article"), discloses a micro-flow-sensor for liquids that operates over a flow range extending from 5 microliters/minute (".mu.l/min") to 500 .mu.l/min. The micro-flow-sensor described in the Gass article employs a cantilevered beam that carries piezo-resistors arranged to form a Wheatstone bridge. Under laminar flow conditions, with a low Reynolds number, the force due to the pressure difference in the liquid on opposite sides of the cantilevered beam is negligible. However, liquid flowing past a free end of the cantilevered beam, and that flow's associated drag on the end of the beam due to viscous shear in the liquid, bends the beam and thereby stresses the piezo-resistors making up the Wheatstone bridge.